I have read many articles about ROC curve. Some specified a method to calculate the accuracy of a classifier using iso accuracy lines in a convex hull.
Just like in this articles :
http://mlwiki.org/index.php/ROC_Analysis

The question is :
Most articles pointed out that, the intersection point between the accuracy line of a classifier (i.e C3) and the diagonal line can be used to determine the accuracy of that classifier, just like in the figure. Why? anyone can prove that for me :(

In addition, given the ratio neg/pos i understood that this ratio is the slope of the accuracy line , but how about the intercept, it is not clear why they can construct a line just with a slope

1 Answer
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As I understand it, the tangent of a 45 degree line is the ‘optimal’ threshold when sensitivity and specificity are equally weighted (often the default). If you change the weighting of sens/spec, you change the slope of the tangent line.

This is not the ‘accuracy’ of the classifier - that’s usually tied to the AUC (accuracy of an ideal observer).

I don’t think the ‘intercept’ of this tangent line has any real interpretation.